Patent GB-933545-A (GB)
Title: Television signal source
IPC: H04N5/225
Filed: 1961-01-06
Abstract: 933,545. Television. FERNSEH G.m.b.H. Jan. 6, 1961 [Jan. 8, 1960], No. 645/61. Class 40 (3). In a television signal source utilizing a pickup tube capable of responding correctly only to images of contrast within a limited range, additional uniform illumination is superimposed upon the image of a scene or object which is projected upon the photo-sensitive member of the pick-up tube and means are arranged when the contrast exceeds said limit to vary the additional illumination so as to reduce the contrast of the image within said range. The invention is of particular application with a tube, such as the image-orthicon and vidicon, and permits such a tube to be used at the same time as another camera, such as a supericonoscope or a kinematograph camera, to view a scene which is lighted with considerable contrast suitable for the other camera. The variation of the additional illumination may be carried out by obtaining a control voltage, preferably by peak rectification, from the video signal produced by the tube. Fig. 4 illustrates the application of the invention to a combined television and kinematograph camera in which the scene to be viewed is imaged by means of an objective lens 18 by way of a partial reflector 19 on the exposure gate of film camera F and is also imaged on the photo-sensitive surface of an image-orthicon tube 25 by means of lenses 22, 24 by way of partial reflectors 23. In accordance with the invention, additional uniform illumination obtained from a source 26 in conjunction with a matt screen and condensing lens 27 is projected through partial reflector 23 so as to superimpose upon the image light reaching tube 25. A light attenuating device 5 is inincluded in the light path to tube 25. Arrangements for adding uniform illumination generally similar to that shown in Fig. 4 are described for use in a camera solely for television, Fig. 1 (not shown), and a film or slide scanner, Fig. 5 (not shown). Another arrangement for adding uniform illumination, Fig. 2 (not shown), comprises a ring of lamps and a cylindrical matt screen surrounding the image optical path immediately following the objective lens. Yet another arrangement, Fig. 3 (not shown), is similar in construction but includes in the light path either a fluorescent liquid confined within the cylindrical screen by two plane-parallel glass plates or a fluorescent glass plate. Reference is also made (without giving details) to the use of a transparent dielectric member making use of the electro-luminescent effect. Preferably the control arrangements are such that when a predetermined contrast is exceeded the amount of the additional illumination is changed by a predetermined amount such as to cause the contrast to be reduced to a second predetermined level. Known electrical means may be employed to equalize the resulting signal contrast in highlights and shadows. The light-attenuating device 5 conveniently comprises two plane-parallel glass plates arranged perpendicular to the axis of the optical path and containing between them a light-absorbing liquid. By altering the separation of the two plates the thickness of light-absorbing liquid in the optical path is varied. Alternatively, an adjustable grey wedge or iris diaphragm may be used.